Some nuclear reactor facilities utilize a liquid metal, such as sodium, as the primary coolant fluid. Such a reactor design has a low pressure and operates at high temperatures. One such nuclear reactor, the so-called "loop type" reactor employs a reactor vessel having a series of looped pipes to provide a path for the flow of hot liquid sodium discharged from the reactor. The hot sodium is sent to heat exchangers which cool the sodium prior to its return to the reactor vessel.
Hot sodium (about 950.degree.-985.degree. F.) causes significant thermal expansion of metal piping. In the loop type reactor the hot radioactive sodium flows through a series of expandable metal loops. The primary disadvantage of a loop type system is cost and maintenance difficulties associated with the large piping systems, supports, trace heating, cell liner systems, and reinforced concrete structures for housing the piping. In addition, the sodium piping exiting the reactor vessel can impose severe transients in the nozzle area which are difficult to accomodate when designing a loop type sodium cooled nuclear reactor facility.
To overcome the disadvantages of using the loop type sodium reactor, there has been developed a reactor facility containing a reactor vessel and at least one satellite vessel which houses the primary heat transport system (e.g. heat exchangers and pumps) and which includes non-looped connecting pipes between the respective vessels. Due to temperature transients arising from the flow of the sodium, the connecting pipes have been provided with expansion bellows.
Such systems typically employ a connecting pipe which enables the hot sodium to flow from the hot plenum of the reactor vessel to the satellite vessel ("the hot leg") and a connecting pipe ("the cold leg") for returning cool sodium from the satellite vessel to the reactor vessel.
The bellows employed in such systems may have to be replaced or repaired because of thermal stress or corrosion which is caused by plant operations or by a faulty bellows unit. Heretofore replacement and repair of the bellows necessitated shutting down the reactor facility and draining the liquid sodium from the reactor and satellite vessels into a storage facility which significantly adds to the cost of supplying nuclear power. The use of valves is also undesirable because they are costly, unreliable and present system design complications.
Usually, a sodium cooled reactor facility is equipped with several storage tanks for storing sodium drained from the reactor facility. However, the use of several storage tanks to house the drained sodium and the need to remove the fuel from the reactor vessel significantly adds to the cost of the operation. It is therefore desirable to devise a reactor facility requiring fewer storage tanks, preferably only a single storage tank, to make repairs within the reactor facility, and which does not require the removal of fuel from the reactor vessel.
The present invention was developed to overcome the problems associated with previous bellows equipped connecting pipes in reactor facilities employing a reactor vessel and at least one satellite vessel to thereby enable easier access to the bellows and to minimize the number of storage tanks. The invention employs non-looped connecting pipes for the flow of hot and cool liquid metal such as sodium between the reactor and the satellite vessels which includes an expanded section containing the bellows assembly.
The bellows assembly used in the present invention allows the connecting pipes to expand and contract with minimal temperature or pressure transients. This is because the connecting pipes are designed to minimize temperature and pressure transients in the vicinity of the openings thereof and the bellows. Furthermore, means are provided for enabling the bellows to be removed for repair or replacement without having to drain the reactor vessel of liquid metal, and thus only a single storage tank is necessary and this is an important factor in reducing the cost of constructing and maintaining the reactor facility.
Still further, the present invention allows several satellite vessels to be directly and closely connected to the reactor vessel at any time durng the lifetime of the facility. That is, at least partial installation of the non-loop connecting pipes may be undertaken during the initial construction phase of the facility. This is desirable if the facility will ultimately operate with several satellite vessels but initially a smaller number of satellite vessels are to be installed and operated.
For example, if a reactor vessel is capable of a 1000 Mwe power level using four satellite vessels, it may be desirable to initially operate the facility at 500 Mwe using two satellite vessels. By providing connecting pipes for four satellite vessels in the initial construction of the reactor facility, the last two satellite vessels may be hooked up in the minimum amount of time without the need for draining liquid metal from the reactor and existing satellite vessels. This can be done by sealing the openings of the connecting pipes which are not to be used initially and then removing the seals when the additional satellite vessels are installed.